For many years, the electric horns commonly used on automotive vehicles have been of the type which generate sound by vibration of a diaphragm driven by an electromagnet motor. The horn typically comprises a housing with the diaphragm peripherally clamped thereto forming a motor chamber. The coil of the electromagnet is mounted within the chamber and a magnetic pole piece on the housing extends axially of the coil. A magnetic plunger on the diaphragm extends toward the pole piece for imparting motion to the diaphragm in response to periodic energization of the coil. The diaphragm provides a resilient suspension of the plunger for reciprocating motion relative to the coil; it has a spring characteristic whereby the diaphragm and the mass carried by it have a resonant frequency of mechanical vibration. The coil is energized from the vehicle battery through a mechanically actuated switch which is alternately opened and closed by movement of the plunger with the diaphragm. A vehicle horn of this kind is described in the Wilson et al U.S. Pat. No. 4,813,123 granted Mar. 21, 1989.
Although vehicle horns of the type just described have been eminently successful in the automotive industry for many years, there have been certain problems which, for a long time, have seemingly defied solution. One such problem is that the life of such a horn is often limited by the life of the switch contacts which are known to deteriorate over long periods of service and lead to failure of the horn. Another such problem is that of manufacturing the horn with sufficiently exacting mechanical and electrical relationships so as to obtain a high degree of operating efficiency. Particularly, such horns have not been readily adjustable to obtain operation at the maximum achievable sound pressure level for a given input power.
A vehicle horn which employs a solid state driver circuit for the horn coil is disclosed and claimed in copending patent application Ser. No. 431,696 filed Nov. 3, 1989 now U.S. Pat. No. 5,049,853 by Y. S. Yoon and assigned to the assignee of this application. In that horn, the driver circuit is adapted to energize the horn coil to cause vibrations of the diaphragm at its resonant frequency. The solid state driver has an electronic timer adjustable to the frequency of the diaphragm assembly and switches a solid state power output stage to drive the diaphragm synchronously with the timer frequency. A driver output stage comprises a power MOSFET or a Darlington pair.
The vehicle horns of the type referred to above, are typically fitted with either a resonant projector or a resonator to propagate sound pressure waves into the atmosphere. The resonant projector is a trumpet-like device comprising a spiral passageway to define an air column of increasing cross-section from the inlet end at the diaphragm to the outlet end at a bell. A horn with this acoustic coupling device is commonly known as a "seashell" horn. It generates sound by the free vibration of the diaphragm. The resonator is a vibratory plate of circular configuration which is mounted at its center on the diaphragm and plunger. In this device, the horn is energized so that the plunger strikes the pole piece during each cycle of diaphragm motion; the force of the strike is transferred to the center of the circular resonator causing it to vibrate at its natural frequency and generate sound pressure waves which are propagated directly into the surrounding atmosphere without any intermediate coupling device. This type of horn is commonly known as a "vibrator" horn. The two horns produce distinctly different sounds. A vehicle is usually provided with a pair of seashell horns or a pair of vibrator horns. To produce the desired sound one horn of each pair is designed for relatively low frequency and the other for high. For the vibrator horns this is typically three hundred fifty hertz and four hundred forty hertz. For seashell horns it is four hundred and five hundred hertz.
In such vehicle horns, it is desired to operate the horn so that the diaphragm is vibrated at its natural resonant frequency. This provides the maximum sound pressure level output from the horn for a given input power. Also, for the purpose of minimizing the power required to drive the horn, it is desired to have the air gap between the plunger and the pole piece at a minimum value consistent with the desired vibrational motion of the diaphragm. For a seashell horn, there is free vibrational motion of the diaphragm, i.e. without any physical contact of the plunger with the pole piece; on the other hand, in the vibrator horn, the vibrational motion of the diaphragm is limited, i.e. the plunger physically strikes the pole piece during each cycle of diaphragm vibration. To achieve this, the stroke length of the plunger must be correlated with the length of air gap which exists between the plunger and pole piece when the diaphragm is at rest.
In the manufacture of vehicle horns of the type having an electromagnet driven diaphragm with a plunger actuated switch contact, it has been a common practice to set the air gap between the plunger and pole piece at a determined length, within manufacturing tolerances, during fabrication of the horn. After assembly the horn is tested and, if necessary, certain adjustments are made. One of the tests, sometimes called the "buzz point" test is for the purpose of determining whether the horn will produce a desired sound quality over the full range of voltage variation likely to be encountered in vehicle operation. In this test the voltage applied to the horn is increased from a value below rated voltage to a value higher than rated voltage. The horn is checked audibly for a "buzz point" voltage, i.e. the voltage at which undesired striking of the plunger against the pole piece occurs. As noted above, no striking is desired for the seashell horn whereas striking with a moderate force is desired for the vibrator horn. An adjusting screw for the switch contacts is adjusted to increase or decrease the time duration of voltage applied to the horn coil. The horn current is also measured during the buzz point test to make sure it is within an acceptable range. If the switch contacts can be adjusted so that the buzz point does not occur when the applied voltage is below a specified value and if the current is not excessive, the horn is acceptable.
The solid state driver circuit set forth in the above-mentioned patent application Ser. No. 431,696 now U.S. Pat. No. 5,049,853, constitutes a significant improvement in respect to elimination of the switch contacts and achieving horn operation with the diaphragm vibrating at its resonant frequency. It allows operation which produces the maximum sound pressure level for a given driving power applied to the horn. However, it does not lend itself to independent adjustment of driving frequency, i.e. pulse repetition rate and input power to the horn. Further, the driving frequency, and driving power varies with changes in the voltage supplied to the horn by the vehicle electrical system.
A general object of this invention is to provide a vehicle horn with a solid state energizing circuit which permits adjustment for operation with high efficiency at maximum sound pressure level and to overcome certain disadvantages of the prior art.